Armor Class (terminology)
From D&D Wiki
How it's used
Armor Class (AC) is used to determine how hard it is to be successfully attacked. It is made up of several components some which reflect your ability to get out of the way, such as your Dexterity bonus, some which make you easier to miss, such as a deflection bonus, and others that represent your ability to get physically hit but not actually injured, such as armour bonus. It is possible to have penalties to your AC as well as bonuses from some of these areas.
Every Creature has three AC values, although some may be identical. Normal which includes everything, Touch which includes only those things that stop physical contact being made and flat footed which includes only those bonuses which work whilst you are not aware of your opponent. Occasionally the flat footed touch Armour Class is also needed.